Anti-skid systems of this type are known, for example from the German Offenlegungsschrift No. 24 60 904 and OS No. 25 43 178. In the former publication, the control frequency of an anti-skid system is altered by varying the ratios of pressure build-up times with different pressure gradients and also possibly by changing the pressure maintenance times in dependence on the pressure increase in a previous cycle. The second publication, OS No. 24 43 178, relates to a supervisory system for an anti-skid system in which two brake pressure control systems are joined whenever one of them has been found to be active for too long a period of time. This system requires additional switches and controls so as to permit a partial or complete inactivation of the control system by suppression of its output signals whenever certain conditions occur.
The anti-skid systems described in the aforementioned publications do not absolutely require the use of braking systems which include pumps and storage chambers but these are well within the scope of the systems described there.
In known braking systems, whether hydraulic or pneumatic, it may happen that when the anti-skid system is required to respond to a highly irregular road surface, the consumption of brake fluid in the anti-skid system is so great that the pump is unable to supply a sufficient amount. If the pump has a diminished volume for other reasons or exhibits a defect of some kind coupled with a possible extremely low external temperature, the primary braking action may be detrimentally affected.